resources: svc: Add basic support for user services

These are user specific services and are available on the session bus.
This doesn't use the private user API because
https://github.com/coreos/go-systemd/pull/225 was NACKed.

examples/svc2.yaml

@@ -0,0 +1,8 @@
+---
+graph: mygraph
+resources:
+  svc:
+  - name: purpleidea
+    state: running
+    session: true
+edges: []